First thing's first here - my sincere thanks to those of you who keep coming back for more, and the web stats tell me there's a lot of you who have stayed dedicated and keep checking in for discussions every day, and the past week and a half or so they have been hard to come by. The situation on that is the combination of being short-staffed here in the weather center, a bunch of behind the scenes projects combined with our usual weather on the ones, the ongoing severe weather coverage of late (which looks to continue for some time) and planning a wedding have conspired to make time for discussion writing limited. I've been contemplating a better way to get information out (quicker on my end) and video is one possibility, but there's something nice about having the ideas written out - it's easier to be sure nothing has been forgotten. So, for the time being I'll crank out as many as I can, as often as I can, and hope to return to daily posts soon, even if they are sometimes brief. I may even transition to a shorter but more frequent update cycle as a flow of thoughts in the true style of a blog...we'll see. Regardless, I appreciate your dedication checking back in.
Now onto the weather. The recent pattern has obviously been a volatile one for New England, with what has become a semi-permanent trough in the Eastern United States - especially the Northeastern US and Southeast Canada - with one upper level low departing and another dropping in. Of course, part of the reason for this has been the persistent ridging over the Western and Central US, which only grows stronger as that area remains hot and dry. No major pattern change lay ahead in the next few weeks - all the way at least through the second and maybe even third week of July - with spurts of warmth and spurts of cool air riding into New England, but moreover an active severe weather pattern through the period with recurring strong and severe thunderstorm events. There comes a point where your amount of damage from these events diminishes due to weeding out the weak trees and limbs with earlier events in the season, but we haven't reached that point yet and the ground has been softening in rain-soaked areas with the repetitive thunderstorms we've seen. This raises the threat for uprooted trees in upcoming severe convective events. Additionally, you can only drop anomalously low heights and upper level cold pools over an area of increasing average surface and boundary layer temperature for so long before something has to give - that is, we've been seeing severe thunderstorms for the past 12 days (day 13 apparently is our lucky number today) somewhere in New England, but this ride may get worse before it gets better. The strength of the vorticity maximums and the magnitude of the cold pools are going to stay just as intense, while the surface warmth and available moisture rises, meaning instability will be greater with each event. This was the pattern in the Midwest this spring, and the result was repetitive and record tornado numbers. Now, I'm not promising the same result for New England, per se, but I think we have to acknowledge that as this same pattern of strong warm/moist advection and persistent, unyielding northern cool air moves over the Northeast and Southeastern Canada, the weather from the Upper Ohio Valley through the Eastern Great Lakes, Southern Ontario/Quebec and New England will get more and more interesting, with perhaps quite a few more twists and turns (literally) than one would normally expect. Of course, we lack the ingredients of the Plains and Midwest from earlier in the year...that was an amazing confluence of every airmass possible...but we still will have sufficient setup for some surprises.
Let's start in the short term. After a gorgeous Wednesday, warm and moist advection that touched off the thunderstorm complex over MO/IL last night will move into the Northeast, likely to instigate nocturnal showers and thunder across PA and NY. Though I expect this action to stay west of New England overnight Wednesday night, we'll have to contend with it on Thursday. Ahead of it, a gradual increase in clouds after midnight and an active southwest wind will keep temps up for much of New England over what they were last night.
Thursday puts parts of the Northeastern US in a precarious position, as we will find warm and moist advection associated with a warm front at all levels making its way northeast into New England Thursday morning. Aloft, this warm advection will mean clouds at and above about 500 mb early in the day. Though this will limit insolation (sunshine) and surface based instability somewhat, the combination of filtered sunshine and an active southerly wind should still work to drive temps up. Standard convective parameters to measure thunderstorm risk, like lifted index, CAPE and Total Totals, are not impressive on Thursday. The initial suggestion, therefore, would be for limited thunderstorms. Remember, though, that this is a warm and moist advection pattern on Thursday, which means it's not your typical "cold over warm" instability setup. Instead, it's an upglide of air flowing from high to low equivalent potential temperature (from warm/moist air into cooler/drier air) and when the low level jet, evidenced at 850 mb, cranks to the tune of 35-50 knots, we're talking about rather intense isentropic lift (rising air induced by air that's migrating across lines of increasing potential temperature). This doesn't depend upon surface based thunderstorm parameters, but rather upon what's available aloft, and the strength of the dynamics at play. Dynamically speaking, a well-defined vort max - several of them, in fact - will traverse New England on Thursday. The first brings the 500 mb clouds and upper level warm advection. Lower in the atmosphere, an impressive mid-level disturbance associated with an attendant wind speed max will prompt development of a low level and surface wave of low pressure. This will enhance warm advection in advance of it on Thursday afternoon, thereby increasing overall lift, then will tug a pocket of cooler and drier air southeast across New England briefly on Thursday evening before the warm and moist advection resumes in earnest, carrying deeper summer heat and humidity into New England on Friday. All the while, though, another shortwave diving southeast over Ontario will move into New England Thursday night, driving a cold front at the surface, and both of these factors will serve to enhance the rising motion of air for New England. This all encourages an active warm front and an active warm sector.
So, the next question is what this means for New England, and while I think we can agree that it will mean convective elements in the warm sector, there are some understandable challenges in taking this a step farther. For instance, will the 500 mb moisture/clouds limit surface based instability enough to reduce mixing and prevent any thunderstorms that develop from interacting with the surface? That seems unlikely with the stiffening wind mixing the atmosphere, except perhaps at the South Coast and Cape Cod, where a wind off the water will mean a shallow cool surface dome. Farther inland and farther west, however, there's plenty in favor of convection. First, of course, is the synoptic situation described above. Next is the increasing warmth and moisture, which by early to middle afternoon is contributing, along with surface heating, to even boost surface based CAPE...climbing to between 500 and 1000 J/kg. More importantly is the enhanced helicity associated with the warm advection that, in Albany, is forecasted to reach over 200. Even without steep lapse rates, the LIs remain below zero but what's most impressive is the veering wind profile with height that shows a classic supercell change with height, the SWEAT index nearing 400...typically the value used as potential tornado development...hardly any convective inhibition, and an energy-helicity index of about 1.15, when I usually like to see over 1.0 for tornadic potential. This setup in Albany holds true for most of Vermont and Western MA, but the parameters decrease quite significantly in Eastern New England. The bottom line here is that all of the factors seem to line up fairly well for an active warm advection convection event, which often can be our most surprising and feature some of our most vigorous storms in the form of supercells. It would seem this potential, with an associated tornadic potential, exists Thursday afternoon in NY/PA and Western New England (esp. VT and Western MA). I wouldn't be surprised to see a tornado watch up for some of these areas from SPC tomorrow, but they - like me - will review the guidance prior to that. There's no question the surface wave development from Central NYS to Central New England enhances the potential by backing the surface flow, putting Western and Southern New England in a precarious spot. Convective potential would be limited in Eastern areas at first, but will increase toward Thursday evening as greater instability sets up ahead of the wave. Remember that the wave carries a small pocket of cooler and drier air from west to east late Thursday and Thursday night from west to east, respectively, then passes by with resumed warm/moist advection Thursday night for perhaps another round of showers, but this sets up very warm/hot air on Friday that will support 90 in some spots with humid air as dewpoints rise into the upper 60s.
Friday's forecast from the GFS looked convectively tainted - the shortwave forecasted to strengthen while moving south of New England and keeping us dry should instead move overhead in the fast and relatively flat flow, so a threat of convection continues Friday afternoon.
That's all for now. Will try to address the weekend thoughts soon.
Matt
Many thanks to SpaceWeather.com for the alert on the following, regarding purple sun (and other colors, too) on the West Coast, resulting from smoke generated by all of the wildfires, and regarding the International Space Station (ISS) flybys that are underway in North America! (Photo courtesy of Christopher Calubaquib and SpaceWeather.com)
